The field of the invention is the field of apparatus for homogenizing laser light for illumination of objects in microscopes and in photolithographic applications.
Lasers may be used as illumination sources for microscopes and for photolithographic systems. Laser beams are typically single mode beams or multimode beams, and the beam homogeneity and coherence properties of such beams may not be sufficient for the application. For single mode laser beams, the light distribution is Gaussian in a line taken perpendicular to the beam. For uniform illumination, the xe2x80x9ctopxe2x80x9d of the beam may be used, but much of the light intensity is then thrown away. Multimode beams may have a xe2x80x9ctop hatxe2x80x9d distribution, but unless all modes have nearly equal population, the intensity may vary over the xe2x80x9ctop hatxe2x80x9d. In applications where intense laser light exposes highly non-linear photoresists, for example, to produce highest resolution exposure in photoresist, a few percent variation in light intensity may be sufficient to degrade the system capability. For microscopic investigations of weak phase objects, such variation may also lead to masking of the small difference in reflected light from neighboring objects, and to introduction of artifacts related to the coherence of light reflected from different parts of the objects in the field of view.
An excellent overview of microscope illuminators is included in S. Inoue, Video Microscopy, Plenum Press, New York, N.Y., 1986.
Prior art systems have used systems of diffusers in the laser beam path to homogenize the beam. Prior art systems such as outlined in an abstract by G. W. Ellis, xe2x80x9cA Fiber-Optic Phase-Randomizer for Microscope Illumination by Laserxe2x80x9d. J. Cell Biol. 83, 303a (1979), have used optical fibers as beam homogenizers to convert near single mode laser beams to multimode beams for illumination systems in microscopes. Prior art systems have used light tunnels, as in U.S. Pat. No. 4,744,615, and systems of lenses, as in U.S. Pat. No. 4,475,027, and holographs, as in U.S. Pat. No. 5,610,733, to homogenize laser beams. The output of such beam homogenizers is often insufficient for exacting lithographic and microscopic work, especially on a small scale. Prior art systems R. D. Allen, xe2x80x9cPhase-Randomized Laser Illumination for Microscopyxe2x80x9d, J. Cell Sci. 23, 335 (1977), have used rotating wedges in combination with a diffuser in a laser beam to move the laser beam around on the object being illuminated. Such rotating wedges average out small scale inhomegenuities in the laser beam, but leave large scale inhomogenuities in place.
It is an object of the invention to produce an apparatus, system, and method for illuminating an object or mask uniformly over a certain area.
It is an object of the invention to produce an apparatus, system, and method for illuminating weak phase object in a microscope with high efficiency.
It is an object of the invention to produce an apparatus, system, and method for illuminating an object or mask uniformly over a certain area with high efficiency, and to record dynamic effects with improved spacial and temporal resolution and contrast.
The present invention is a system, apparatus and method for uniformly illuminating a field, (such as an object in an object plane or the rear focal plane of an epi illuminating objective lens) by imaging the output aperture of a laser or other light beam homogenizer on the field using an apparatus which dithers the image of the output of the beam homogenizer with respect to the field perpendicularly to the optical axis or dithers the output of the beam homogenizer so that the image of the beam homogenizer dithers with respect to the object or the rear focal plane. The dither means, in a preferred embodiment, is synchronized with dynamic effects in the object or with a video recording device to provide enhanced resolution.